Japanese Patent Publication (JP-A-Heisei 10-258463) discloses a rib of composite material, which is used in awing of an aircraft. As shown in FIG. 1, a rib 101 includes a web 102 and flanges 103 provided at both ends of the web 102. In order to increase the rigidity of the rib 101, a plurality of beads 104 are provided along the longitudinal direction of the web 102. Each of the beads 104 extends in a direction orthogonal to the longitudinal direction. Since the beads 104 are provided, an actual length L101 of the web 102 shown in FIG. 2 is longer than an actual length L102 of the flange 103 shown in FIG. 1.
When the rib 101 is molded by using a prepreg material including woven fabric of carbon or glass fiber, wrinkles tend to be generated at corner portions of the bead 104. This is because, since the prepreg material cannot be stretched in the direction of fiber, a difference between the actual length L101 and the actual length L102 cannot be absorbed.
As shown in FIG. 3, a method is known in which the difference between the actual length L101 and the actual length L102 is absorbed by forming cuts 106 in a prepreg material 105 at locations corresponding to the corner portions of the beads 104.